This is an amendment to SPID 3530. The major aim of this study is to carry out a prospective randomized clinical trial of deep brain stimulation in the internal segment of the globus pallidus and subthalamic nucleus for the treatment of advanced Parkinson's disease. PET imaging will be used to understand how the brain circuits involved in controlling movement are impaired, and how activity in those circuits is changed following surgical treatment for Parkinson's disease.